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72 lines
2.1 KiB
C
72 lines
2.1 KiB
C
/* Compute cubic root of double value.
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Copyright (C) 1997-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Dirk Alboth <dirka@uni-paderborn.de> and
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Ulrich Drepper <drepper@cygnus.com>, 1997.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <math.h>
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#include <math_private.h>
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#include <libm-alias-ldouble.h>
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#define CBRT2 1.2599210498948731648 /* 2^(1/3) */
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#define SQR_CBRT2 1.5874010519681994748 /* 2^(2/3) */
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/* We don't use long double values here since U need not be computed
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with full precision. */
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static const double factor[5] =
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{
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1.0 / SQR_CBRT2,
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1.0 / CBRT2,
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1.0,
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CBRT2,
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SQR_CBRT2
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};
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static const long double third = 0.3333333333333333333333333L;
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long double
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__cbrtl (long double x)
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{
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long double xm, u;
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int xe;
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/* Reduce X. XM now is an range 1.0 to 0.5. */
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xm = __frexpl (fabsl (x), &xe);
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/* If X is not finite or is null return it (with raising exceptions
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if necessary.
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Note: *Our* version of `frexp' sets XE to zero if the argument is
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Inf or NaN. This is not portable but faster. */
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if (xe == 0 && fpclassify (x) <= FP_ZERO)
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return x + x;
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u = (((-1.34661104733595206551E-1 * xm
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+ 5.46646013663955245034E-1) * xm
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- 9.54382247715094465250E-1) * xm
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+ 1.13999833547172932737E0) * xm
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+ 4.02389795645447521269E-1;
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u *= factor[2 + xe % 3];
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u = __ldexpl (x > 0.0 ? u : -u, xe / 3);
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u -= (u - (x / (u * u))) * third;
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u -= (u - (x / (u * u))) * third;
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return u;
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}
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libm_alias_ldouble (__cbrt, cbrt)
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